Sheet conveyance apparatus and image forming apparatus

ABSTRACT

A sheet conveyance apparatus includes a first rotary member pair comprising a first nip portion and configured to convey a sheet, a second rotary member pair comprising a second nip portion and configured to convey the sheet being conveyed by the first rotary member pair, and a conveyance guide including a first conveyance guide portion forming a loop space and a second conveyance guide portion arranged downstream of the first conveyance guide portion in the sheet conveyance direction and forming a curved conveyance path. A first intersection at which a first straight line and a third straight line intersect is positioned within the loop space, and a second intersection at which the first straight line and a second straight line intersect is positioned outside the conveyance path.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a sheet conveyance apparatus forconveying sheets and an image forming apparatus equipped with the same.

Description of the Related Art

Generally, in image forming apparatuses such as printers, a sheet fedfrom a sheet feeding unit is abutted against a nip of a registrationroller pair whose rotation is stopped, to thereby form a loop on thesheet and correct skewing of the sheet. Thereafter, the registrationroller pair rotates to convey the sheet to an image forming portion, andan image is formed on the sheet at the image forming portion.Conventionally, Japanese Patent Laid-Open Publication No. 2000-118801proposes an image forming apparatus in which an amount of loop formed onthe sheet is adjusted based on sheet type information. For example, theimage forming apparatus controls the registration roller pair to form agreater loop in a normal paper having relatively weak stiffness comparedto a sheet having greater stiffness such as envelope, postcard, thickpaper and OHP sheet.

According to the printer taught in Japanese Patent Laid-Open PublicationNo. 2000-118801, in a state where the sheet subjected to skew correctionis conveyed by the registration roller pair, a trailing edge side of thesheet is still skewed. Therefore, torsion occurs in the loop formed in aspace between the registration roller pair and an upstream roller pairarranged upstream in a conveyance direction thereof. If the sheet isconveyed by the registration roller pair and the upstream roller pair ina state where there is torsion in the loop, shearing force acting on thesheet increases gradually. If the shearing force acting on the sheetexceeds the rigidity of the sheet, the sheet is buckled and creases areformed on the sheet when the sheet passes the registration roller pair.Especially if a distance between the registration roller pair and theupstream roller pair is narrow, the sheet tends to be buckled.

Recently, there are demands for image forming apparatuses thatcorrespond to various types of media such as a long-size sheet, asmall-size sheet, thin paper and coated paper. In order to convey asheet having a high rigidity, it is necessary to set the registrationroller pair and the upstream roller pair to have a high nip pressure,and in order to convey a small-size sheet, it is necessary to set thedistance between the registration roller pair and the upstream rollerpair to be narrow. If a thin paper having low rigidity or a long-sizedsheet is conveyed by the registration roller pair and the upstreamroller pair, the shearing force applied on the sheet may exceed therigidity of the sheet, and the sheet may be buckled.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a sheet conveyanceapparatus includes a first rotary member pair comprising a first nipportion and configured to convey a sheet, a second rotary member pairarranged downstream of the first rotary member pair in a sheetconveyance direction, comprising a second nip portion and configured toconvey the sheet being conveyed by the first rotary member pair, and aconveyance guide configured to guide the sheet having passed through thefirst nip portion to the second nip portion, and forming a curvedconveyance path, the conveyance guide including a first conveyance guideportion forming a loop space and a second conveyance guide portionarranged downstream of the first conveyance guide portion in the sheetconveyance direction, the conveyance path including the loop space inwhich a sheet abutted against the second nip portion is capable offorming a loop. In the conveyance path, the first rotary member pair andthe second rotary member pair are arranged adjacently. The second nipportion is positioned upward than the first nip portion in a verticaldirection and positioned at a position different from the first nipportion in a horizontal direction. The conveyance guide is arranged onan outer side of the curved conveyance path between the first nipportion and the second nip portion. The sheet conveyed by the firstrotary member pair is abutted against the second nip portion so as tocorrect skewing of the sheet. If a common tangent of the first rotarymember pair at the first nip portion is referred to as a first straightline, a common tangent of the second rotary member pair at the secondnip portion is referred to as a second straight line, and a straightline that passes an intersection of the second straight line and theconveyance guide and has an inclination corresponding to the secondconveyance guide portion is referred to as a third straight line, thesecond straight line and the third straight line intersect each other,the loop space is arranged upstream of the intersection of the secondstraight line and the conveyance guide in the sheet conveyancedirection, a first intersection at which the first straight line and thethird straight line intersect is positioned within the loop space, and asecond intersection at which the first straight line and the secondstraight line intersect is positioned outside the conveyance path.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall schematic view illustrating a printer according toa present embodiment.

FIG. 2 is a schematic diagram illustrating a manual sheet feed apparatusand a skewing correction apparatus.

FIG. 3 is a control block diagram illustrating a controller.

FIG. 4 is a flowchart illustrating a sheet conveyance control.

FIG. 5 is a graph showing a positional relationship between respectiverollers and the sheet.

FIG. 6A is a view illustrating a state in which a leading edge of thesheet is abutted against a nip of a registration roller pair.

FIG. 6B is a view illustrating a state in which the sheet has formed aloop.

FIG. 6C is a view illustrating a state in which a sheet is conveyed bythe registration roller pair and a pre-registration roller pair.

FIG. 7A is a view illustrating a state in which a shearing force actingon the sheet escapes.

FIG. 7B illustrates a state in which a trailing edge of the sheet haspassed through the pre-registration roller pair.

FIG. 8A is a view illustrating a conveyance guide according to a firstmodified example.

FIG. 8B is a view illustrating a conveyance guide according to a secondmodified example.

FIG. 9 is a view illustrating a skewing correction apparatus accordingto a comparative example.

FIG. 10 is a view illustrating torsion acting on the sheet.

DESCRIPTION OF THE EMBODIMENTS

Now, a preferred embodiment of the present embodiment will be describedwith reference to the drawings. In the following description, thepositional relationship of the image forming apparatus in up-down,right-left and front-rear directions will be described based on a statein which the image forming apparatus is viewed from the front side, thatis, viewpoint of FIG. 1.

Image Forming Apparatus

A printer 201 serving as an image forming apparatus according to apresent embodiment is an electro-photographic full-color laser beamprinter. The printer 201 includes, as illustrated in FIG. 1, a printerbody 201A serving as an apparatus body, and a reading unit disposedabove the printer body 201A for reading image data on a document.

The printer body 201A includes an image forming portion 201B for formingan image on a sheet P, a fixing portion 220 for fixing an image on thesheet P, and so on. A sheet discharge space to which the sheet P isdischarged is formed between the reading unit 202 and the printer body201A, and a sheet discharge tray 230 supporting the discharged sheets Pis arranged in the sheet discharge space. Further, a sheet feeding unit201E for feeding sheets P to the image forming portion 201B is providedon the printer body 201A. The sheet feeding unit 201E includes sheetfeed cassettes 100A, 100B, 100C and 100D arranged at a lower portion ofthe printer body 201A and storing sheets P in cassettes 103, and amanual sheet feed apparatus 100M arranged on a right side portion of theprinter body 201A. Each of the sheet feed cassettes 100A, 100B, 100C and100D and the manual sheet feed apparatus 100M includes a pickup roller 2for feeding sheets P, and a feed roller 3 and a retard roller 4 forseparating the sheet P one by one and conveying the same.

The image forming portion 201B is a so-called four-drum full color imageforming portion that includes a laser scanner 210, four processcartridges 211, and an intermediate transfer unit 201C. The processcartridges respectively form toner images of different colors, which areyellow (Y), magenta (M), cyan (C) and black (K). Each process cartridge211 includes a photosensitive drum 212, a charger 213, a developer 214,a cleaner not shown and so on. A toner cartridge 215 storing toner ofrespective colors is removably attached to the printer body 201A at anarea above the image forming portion 201B.

The intermediate transfer unit 201C includes an intermediate transferbelt 216 wound around a drive roller 216 a, a tension roller 216 b andso on, and the intermediate transfer belt 216 is arranged above the fourprocess cartridges 211. The intermediate transfer belt 216 is arrangedto contact the photosensitive drums 212 of the respective processcartridges 211, and driven to rotate in a counterclockwise direction,that is, direction of arrow Q, by the drive roller 216 a which is drivenby a driving unit not shown. The intermediate transfer unit 201Cincludes primary transfer rollers 219 that contact an innercircumferential surface of the intermediate transfer belt 216 atpositions opposed to respective photosensitive drums 212, and a primarytransfer portion TP1 is formed as a nip portion between the intermediatetransfer belt 216 and the photosensitive drum 212. Further, the imageforming portion 201B includes a secondary transfer roller 217 thatcontacts an outer circumference surface of the intermediate transferbelt 216 at a position opposed to the drive roller 216 a. A secondarytransfer portion TP2 is formed as a nip portion between the secondarytransfer roller 217 and the intermediate transfer belt 216 where a tonerimage borne on the intermediate transfer belt 216 is transferred onto asheet P.

In the respective process cartridges 211 configured as described above,an electrostatic latent image is formed on the surface of thephotosensitive drum 212 by the laser scanner 210, and thereafter, toneris supplied from the developer 214 so that toner images of respectivecolors charged to negative polarity are formed. The toner images aresequentially transferred in multilayers on the intermediate transferbelt 216 at the respective primary transfer portions TP1 by havingtransfer bias voltage of positive polarity applied to the primarytransfer roller 219, and a full color toner image is formed on theintermediate transfer belt 216.

In parallel with the above-described image forming process, the sheet Pfed from the sheet feeding unit 201E is conveyed toward a registrationroller pair 15 and skewing of the sheet P is corrected by theregistration roller pair 15. The registration roller pair 15 conveys thesheet P to the secondary transfer portion TP2 at a matched timing withthe transfer timing of the full color toner image formed on theintermediate transfer belt 216. The toner image borne on theintermediate transfer belt 216 is secondarily transferred to the sheet Pat the secondary transfer portion TP2 by having transfer bias voltage ofpositive polarity applied to the secondary transfer roller 217.

The sheet P to which the toner image has been transferred is heated andpressed at the fixing portion 220, and the color image is fixed to thesheet P. The sheet P on which the image has been fixed is discharged bya sheet discharge roller pair 225 to the sheet discharge tray 230 andsupported thereon. If it is necessary to form images on both sides ofthe sheet P, the sheet P having passed the fixing portion 220 isswitched back by a reverse conveyance roller pair 222 that is capable ofrotating in normal and reverse directions provided in a reverseconveyance portion 201D. Thereafter, the sheet P is conveyed again tothe secondary transfer portion TP2 through a re-conveyance path R, andan image is formed on the rear side of the sheet P.

Manual Sheet Feed Apparatus and Skewing Correction Apparatus

Next, with reference to FIG. 2, we will describe the manual sheet feedapparatus 100M, and a skewing correction apparatus 100R including theregistration roller pair 15. The pickup roller 2, the feed roller 3 andthe retard roller 4 of the manual sheet feed apparatus 100M areconnected to a sheet feed motor M1, and a torque limiter not shown isarranged between the sheet feed motor M1 and the retard roller 4. Thepickup roller 2 and the feed roller 3 receive drive force from the sheetfeed motor M1 that rotates the rollers in a direction conveying thesheet P toward the skewing correction apparatus 100R, and the retardroller 4 receives drive force toward an opposite direction from thesheet feed motor M1. In order to feed the sheet P supported on a manualsheet feed tray 7, the sheet feed motor M1 is driven in a state wherethe pickup roller 2 contacts the sheet P with a predetermined pressure.

The sheet P fed by the pickup roller 2 is conveyed to a separation nip 3a formed by the feed roller 3 and the retard roller 4, and the sheet isseparated one by one at the separation nip 3 a. That is, if no sheet Por one sheet P enters the separation nip 3 a, the torque limiter rotatesidly, and the retard roller 4 rotates following the rotation of the feedroller 3, conveying the sheet P toward the skewing correction apparatus100R. In a state where two or more sheets P enter the separation nip 3a, the friction between the plural sheets P is small, such that theretard roller 4 rotates toward a direction returning the sheet P towardthe manual sheet feed tray 7 and separates the multiple-transferredsheet P one by one.

The sheet P separated one by one at the separation nip 3 a is conveyedto a pre-registration roller pair 8. The pre-registration roller pair 8serving as a first rotary member pair is connected to a pre-registrationmotor M2, and the pre-registration roller pair 8 is rotated by drivingthe pre-registration motor M2. The sheet P is conveyed toward aconveyance guide 18 by a nip 8 a serving as a first nip portion of thepre-registration roller pair 8 and guided by the conveyance guide 18 tothe registration roller pair 15. A registration sensor 12 is arrangedupstream in a sheet conveyance direction of a nip 15 a serving as asecond nip portion of the registration roller pair 15, and theregistration sensor 12 detects a leading edge position of the sheet P.

The registration roller pair 15 serving as a second rotary member pairis connected to a registration motor M3, and in a state where theleading edge of the sheet P reaches the nip 15 a of the registrationroller pair 15, the registration motor M3 is stopped. Therefore, theleading edge of the sheet P is abutted against the nip 15 a of theregistration roller pair 15 in the stopped state. In this state, thesheet P is further conveyed by the pre-registration roller pair 8, suchthat the sheet P forms a loop and skewing is corrected. The registrationroller pair 15 starts to rotate after being stopped for a predeterminedtime, and the sheet P is conveyed by the registration roller pair 15 andthe pre-registration roller pair 8 toward the secondary transfer portionTP2. Since skewing of the sheet P is corrected by the skewing correctionapparatus 100R, the sheet P and the image transferred to the sheet P atthe secondary transfer portion TP2 will not be skewed or laterallydislocated, and a good product can be obtained.

Controller

As illustrated in FIG. 3, a size detection sensor 11 and theregistration sensor 12 are connected to an input side of a controller 9of the printer 201. The size detection sensor 11 detects the size of thesheet P supported on the manual sheet feed tray 7 or a cassette 103. Thesheet feed motor M1, the pre-registration motor M2 and the registrationmotor M3 are connected to an output side of the controller 9. Further, atimer 13 is connected to the controller 9.

Conveyance Control of Sheet

Next, conveyance control of the sheet P according to the presentembodiment will be described. FIG. 4 is a flowchart illustratingconveyance control of the sheet P FIG. 5 is a graph illustratingtheoretical positions of a leading edge and a trailing edge of the sheetat the pickup roller 2, the pre-registration roller pair 8, theregistration sensor 12, the registration roller pair 15 and thesecondary transfer roller 217. Line A illustrates a theoretical lineindicating the tip position of the sheet P, and line B illustrates atheoretical line indicating a trailing edge position of the sheet P.Lines D, E and F respectively illustrate peripheral drive speeds of thepickup roller 2, the pre-registration roller pair 8 and the registrationroller pair 15. The theoretical line B illustrating the trailing edgeposition of the sheet P can be calculated based on the theoretical lineA of the tip position of the sheet P and a length L of the sheet Pdetected by the size detection sensor 11.

As illustrated in FIG. 4, if the sheet P is supported on the manualsheet feed tray 7, the controller 9 detects the size of the fed sheet Pby the size detection sensor 11 (step S1). If the size of the sheet P isdetected (step S1: YES), the controller 9 determines a length L of thesheet P (step S2). The controller 9 drives the sheet feed motor M1 andthe pre-registration motor M2 (step S3). At this time, the peripheraldrive speed of the pickup roller 2 and the pre-registration roller pair8 are set to a sheet feed speed V1.

If the leading edge of the sheet P reaches the nip 8 a of thepre-registration roller pair 8, the controller 9 stops the sheet feedmotor M1 and stops the pickup roller 2. In this state, the pickup roller2 can be separated from the sheet P by a swing arm not shown. Accordingto another example, an intermediate plate for supporting a sheet can beprovided on the manual sheet feed tray 7, and the intermediate plate canbe moved downward. According to another example, a one-way clutch can beprovided to the pickup roller 2.

Then, the sheet P is conveyed toward the registration sensor 12 by thepre-registration roller pair 8. The controller 9 measures a controltime, which is an elapsed time from when drive of the pickup roller 2has been started, using a timer 13, and determines whether a leadingedge of the sheet P has been detected by the registration sensor 12within a control time T1 calculated in advance (step S4). If the leadingedge of the sheet P is not detected within the control time T1 (step S4:NO), the controller 9 determines that delay jam has occurred (step S5).If it is determined that delay jam has occurred, the user is notifiedthat jam has occurred by an alarm sound and a display on an operationpanel.

If the leading edge of the sheet P is detected within the control timeT1 by the registration sensor 12 (step S4: YES), the controller 9 stopsthe pre-registration motor M2 at control time T2 (step S6). The leadingedge of the sheet P reaches the nip 8 a of the pre-registration rollerpair 8 between control time T1 and control time T2, and thepre-registration motor M2 is continued to be driven even after theleading edge has reached the nip 8 a, so that the sheet P forms a loop.The controller 9 sets the control time T2 so that an amount of loopcalculated in advance is formed on the sheet P.

After stopping the pre-registration motor M2 for a predetermined time soas to synchronize with the transfer timing at the secondary transferportion TP2, the controller 9 drives the pre-registration motor M2 andthe registration motor M3 at control time T3 (step S7). Thereby, thepre-registration roller pair 8 and the registration roller pair 15 arerotated at image forming speed V2, and the sheet P is conveyed towardthe secondary transfer portion TP2. The image forming speed V2 is equalto a rotational speed of the intermediate transfer belt 216.

The pre-registration roller pair 8 does not have to be driven at thesame timing as the registration roller pair 15 at control time T3, anddriving of the pre-registration roller pair 8 can be started at a latertiming than the registration roller pair 15 as long as it is startedbefore the loop formed on the sheet P is dissolved. That is, driving ofthe pre-registration roller pair 8 can be started on or before controltime T4, which is a time having added time X1 to control time T3, timeX1 being the time when the registration roller pair 15 dissolves theloop formed on the sheet Pin a state where the pre-registration rollerpair 8 is stopped.

Then, the controller 9 stops the pre-registration motor M2 at controltime T5 when the trailing edge of the sheet P passes thepre-registration roller pair 8 and stops the rotation of thepre-registration roller pair 8 (step S8). Further, the controller 9stops the registration motor M3 at control time T6 when the trailingedge of the sheet P passes the registration roller pair 15 and stops therotation of the registration roller pair 15 (step S9). Thereby,conveyance control of the sheet P is completed.

The control times T5 and T6 are not restricted to the times at which thetrailing edge of the sheet passes the pre-registration roller pair 8 andthe registration roller pair 15, and the times can be somewhat variedwithin a range that does not affect the conveyance of the sheet PFurther, conveyance control of the sheet P according to the presentembodiment illustrates a case where the sheet feed speed V1 isapproximately 300 mm/sec and the image forming speed V2 is approximately250 mm/sec, but the sheet feed speed V1 and the image forming speed V2can be varied arbitrarily.

Comparative Example

Now, a skewing correction apparatus 101R according to a comparativeexample will be described with reference to FIGS. 9 and 10. Asillustrated in FIG. 9, the skewing correction apparatus 101R includes apre-registration roller pair 8, a registration roller pair 15 and aconveyance guide 118 provided between the pre-registration roller pair 8and the registration roller pair 15.

The conveyance guide 118 includes a guide surface 118 a that extends inparallel with a nip line NL2, which is a common tangent of theregistration roller pair 15 at the nip 15 a, and guides the sheet P tothe nip 15 a. Further, the conveyance guide 118 includes a bulgedportion 118 c that curves from an upstream end 118 b of the guidesurface 118 a in a sheet conveyance direction, and bulges toward adirection away from a nip line NL1, which is a common tangent of thepre-registration roller pair 8 at the nip 8 a. The bulged portion 118 cforms a loop space 119 in which the sheet P is capable of forming aloop. Further, a first opposing guide 20 and a second opposing guide 23are arranged to oppose to the conveyance guide 118, and a curvedconveyance path CP1, including the loop space 119, is formed by theconveyance guide 118, the first opposing guide 20 and the secondopposing guide 23.

The nip line NL1 and the nip line NL2 intersect at an intersection 132on the guide surface 118 a of the conveyance guide 118. That is, the nipline NL1 intersects with the guide surface 118 a. Thereby, the sheet Pconveyed to a direction parallel to the nip line NL1 by thepre-registration roller pair 8 contacts the guide surface 118 a of theconveyance guide 118 near the intersection 132 and changes its path to adirection parallel to the nip line NL2 along the guide surface 118 a. InFIG. 9, the nip line NL2 and the guide surface 118 a are illustrated asif a slight gap is formed therebetween, but the gap is merelyillustrated for the sake of explanation, and actually, the nip line NL2is overlapped with the guide surface 118 a.

The sheet P forms a loop within the loop space 119 by the leading edgeof the sheet P abutting against the nip 15 a of the registration rollerpair 15, and by contact force of the sheet P and the conveyance guide118, sagging of the sheet by gravity can be prevented even if therigidity of the sheet P is low. Therefore, conveyance force of thepre-registration roller pair 8 is transmitted to the leading edge of thesheet P, and the leading edge of the sheet P is pressed against the nip15 a, by which skewing is corrected reliably.

In FIG. 10, the sheet P having its leading edge abutted against the nip15 a of the registration roller pair 15 and forming a loop isillustrated by a broken line, and the sheet P whose skewing is correctedand conveyed by the registration roller pair 15 and the pre-registrationroller pair 8 is illustrated by a solid line. The sheet P illustrated bythe broken line is subjected to skew feed control at the nip 15 a at theleading edge side, but the trailing edge side of the sheet P is stillskewed, so that torsion is generated in the loop formed between theregistration roller pair 15 and the pre-registration roller pair 8.Therefore, if the sheet P is continued to be conveyed by theregistration roller pair 15 and the pre-registration roller pair 8,shearing force acting on the sheet P is gradually increased.

Since the shearing force applied on the sheet P is especially appliednear the intersection 132, if the shearing force exceeds the rigidity ofthe sheet P, the sheet P is buckled from the area close to theintersection 132, and when the sheet P passes through the nip 15 a ofthe registration roller pair 15, creases PW tend to be formed on thesheet P. Creases PW tend to be formed on a sheet having a long size orhaving a low rigidity. Further, creases PW are significantly formed in acase where the distance between two sets of rollers for forming a loopon the sheet is set short or in a case where conveyance force of the twosets of rollers is set high.

Skewing Correction Apparatus

Next, the skewing correction apparatus 100R according to the presentembodiment will be described with reference to FIG. 2. The skewingcorrection apparatus 100R includes the pre-registration roller pair 8,the registration roller pair 15, and the conveyance guide 18 providedbetween the pre-registration roller pair 8 and the registration rollerpair 15. The pre-registration roller pair 8 is composed of a pair ofrubber rollers, and nip pressure of the pre-registration roller pair 8is set high so as to convey a sheet having a relatively high grammage,such as thick paper. The conveyance guide 18 has a different shape asthe conveyance guide 118 according to the above-described comparativeexample.

The nip 15 a of the registration roller pair 15 is positioned upwardthan the nip 8 a of the pre-registration roller pair 8 in the verticaldirection and at a position that differs from the nip 8 a in thehorizontal direction. The conveyance guide 18 is provided on an outerside of a curved conveyance path CP between the nip 8 a and the nip 15a. The nip line NL2 serving as a second straight line, which is a commontangent of the registration roller pair 15 at the nip 15 a intersectswith a guide surface 18 a serving as a second conveyance guide portionat an intersection 31. The guide surface 18 a guides the sheet P to thenip 15 a. The intersection 31 is separated by distance h2 from the nip15 a in a direction parallel to the nip line NL2. Further, theconveyance guide 18 includes a bulged portion 18 c bulging toward adirection away from the nip line NL1 as a first straight line which is acommon tangent of the pre-registration roller pair 8 at the nip 8 a. Thebulged portion 18 c, serving as a first conveyance guide portion, isformed in a curved shape and formed continuously with an upstream end 18b, in the sheet conveyance direction, of the guide surface 18 a. It isnoted that the bulged portion 18 c is provided so as to intersect withthe nip line NL1. The guide surface 18 a is a flat surface which isformed linearly and non-parallelly with the nip lines NL1 and NL2between the upstream end 18 b and a downstream end 18 d. The bulgedportion 18 c forms a loop space 19 in which the sheet P can form a loop.Further, the first opposing guide 20 and the second opposing guide 23are arranged to oppose to the conveyance guide 18, and a conveyance pathCP including the loop space 19 is formed by the conveyance guide 18, thefirst opposing guide 20 and the second opposing guide 23. Thepre-registration roller pair 8 and the registration roller pair 15 arearranged adjacently in the conveyance path CP.

If a straight line that passes the intersection 31 and has aninclination corresponding to the guide surface 18 a, that is, if atangent which is tangent to the conveyance guide 18 at the intersection31 is referred to as a straight line TL2, the straight line TL2 servingas a third straight line is inclined by minor angle θ₁ with respect tothe nip line NL2. That is, the nip line NL2 and the straight line TL2intersect. A minor angle is a smaller angle of angles formed by twostraight lines, and it is equal to or smaller than 90 degrees. Further,the straight line TL2 is inclined by minor angle θ₂, that is greaterthan minor angle θ₁, with respect to the nip line NL1 of thepre-registration roller pair 8. That is, the minor angle θ₂ formed bythe nip line NL1 serving as the first straight line and the straightline TL2 is greater than the minor angle θ₁ formed by the nip line NL2serving as the second straight line and the straight line TL2 (θ₂>θ₁).Further, an inclined surface 19 a downstream of the bulged portion 18 cformed from the upstream end 18 b of the guide surface 18 a extendsalong the straight line TL1, and the straight line TL2 is inclined byminor angle θ₃ with respect to the straight line TL1. The loop space 19is a space that is sufficiently greater than a pre-registrationconveyance space 21 serving as conveyance space between the conveyanceguide 18 and the registration roller pair 15. The nip 8 a and 15 a arepositioned on one side with respect to the straight line TL2.

It is preferable to set the minor angle θ₁ to approximately 10°, theminor angle θ₂ to approximately 20°, the minor angle θ₃ to approximately15°, the distance h2 to approximately 10 mm and the length of the guidesurface 18 a to approximately 15 to 20 mm, but the numerical values arenot restricted thereto and can be varied arbitrarily.

An intersection 32 serving as a first intersection between the nip lineNL1 and the straight line TL2 is separated by a distance h1 in adirection parallel with the straight line TL2 from the upstream end 18 bof the guide surface 18 a, and it is positioned within the loop space19. That is, the intersection 32 is positioned within the conveyancepath CP. Further, the nip line NL1 intersects with the inclined surface19 a of the bulged portion 18 c, and an intersection 33 serving as asecond intersection in which the nip line NL1 and the nip line NL2intersect is positioned outside the conveyance path CP through which thesheet passes.

Skewing Correction Operation

Next, skewing correction operation of the sheet P by a skewingcorrection apparatus 100R according to the present embodiment will bedescribed with reference to FIGS. 6A through 7B. The sheet P conveyed bythe pre-registration roller pair 8 is conveyed to a direction parallelto the nip line NL1 and contacts the inclined surface 19 a of the bulgedportion 18 c, and changes its course along the inclined surface 19 a.Further, the sheet P is transferred from the inclined surface 19 a tothe guide surface 18 a, and the sheet P is guided along the guidesurface 18 a to a direction parallel to the straight line TL2. That is,the leading edge of the sheet conveyed by the pre-registration rollerpair 8 is guided to the guide surface 18 a by the inclined surface 19 aof the bulged portion 18 c, and thereafter, guided by the guide surface18 a to the nip 15 a of the registration roller pair 15.

If the sheet P is further conveyed by the pre-registration roller pair8, as illustrated in FIG. 6A, the leading edge of the sheet P is abuttedagainst the nip 15 a of the registration roller pair 15 and the leadingedge of the sheet P faces a direction parallel to the nip line NL2. Inthis state, a reaction force F1 acting on the leading edge of the sheetP from the nip 15 a and a contact force F2 between the sheet P and theguide surface 18 a are balanced. Further, the guide surface 18 a extendsin a direction inclined by angle θ₄ with respect to a gravity direction(refer to FIG. 2), contacts the sheet sagged by its own weight, andreceives load from the sheet.

As illustrated in FIG. 6B, if the sheet P is conveyed further by thepre-registration roller pair 8, the sheet P forms a loop that originatesfrom an area close to the intersection 32. In this state, a reactionforce F3 acting on the leading edge of the sheet P from the nip 15 a anda contact force F4 between the sheet P and the guide surface 18 a arebalanced. Further, contact force between the sheet P and the guidesurface 18 a of the conveyance guide 18 prevents sagging of the sheet bygravity even if the rigidity of the sheet P is low. Therefore,conveyance force of the pre-registration roller pair 8 can betransmitted to the leading edge of the sheet P, and the leading edge ofthe sheet P can be pressed against the nip 15 a, by which skewing can becorrected reliably.

As illustrated in FIG. 6C, in a state where the registration roller pair15 starts to rotate and the sheet P is conveyed by the registrationroller pair 15 and the pre-registration roller pair 8, the leading edgeside of the sheet is conveyed in a state where skewing is correctedalong the nip 15 a. However, the trailing edge side of the sheet isstill skewed, such that torsion is generated in the loop formed betweenthe registration roller pair 15 and the pre-registration roller pair 8.In this state, a reaction force F5 applied on the sheet P from the nip15 a and a contact force F6 between the sheet P and the guide surface 18a are still balanced.

As illustrated in FIG. 7A, if the sheet P is conveyed further by theregistration roller pair 15 and the pre-registration roller pair 8, theshearing force acting on the sheet P is increased gradually. If reactionforce F7 acting on the sheet P from the nip 15 a exceeds a contact forceF8 between the sheet P and the guide surface 18 a, the sheet P slips onthe guide surface 18 a. Thereby, shearing force F9 which is an excess ofthe reaction force F7 having exceeded the contact force F8 can bereleased to the loop formed in the loop space 19 originating from theintersection 32, and creasing of the sheet P can be prevented. Asillustrated in FIG. 7B, if the trailing edge of the sheet P passesthrough the nip 8 a of the pre-registration roller pair 8, the shearingforce acting on the sheet P and the torsion of the loop are released.

As described, as illustrated in FIG. 2, according to the presentembodiment, the nip line NL2 of the registration roller pair 15 and theguide surface 18 a of the conveyance guide 18 intersect at theintersection 31. Therefore, in a state where the sheet abuts against thenip 8 a and forms a loop, the contact force of the sheet P and the guidesurface 18 a enables to press the leading edge of the sheet against thenip 8 a, and skewing of even a sheet having low rigidity, such as thinpaper, can be corrected reliably.

Further, the straight line TL2 and the nip line NL1 of thepre-registration roller pair 8 intersect at the intersection 32 withinthe loop space 19 distant from the conveyance guide 18. Therefore, in astate where the sheet in which a loop is formed is conveyed by theregistration roller pair 15 and the pre-registration roller pair 8, evenif the shearing force acting on the sheet is increased, the shearingforce can be released to the loop originated from the intersection 32.Thus, buckling and creasing of the sheet can be prevented.

Since the loop space 19 is sufficiently greater than thepre-registration conveyance space 21 formed between the conveyance guide18 and the registration roller pair 15, the loop of the sheet is mainlyformed within the loop space 19, and shearing force can be releasedreliably within the loop space 19.

As described, the embodiment is configured such that during conveyanceof the sheet P, the sheet P is bent in two steps, one originating fromthe intersection 31 and the other originating from the intersection 32,so that even if the distance between the registration roller pair 15 andthe pre-registration roller pair 8 is narrow, skewing of the sheet canbe corrected reliably and creasing can be prevented. Especially in asheet having low rigidity and long size where the conveyance distance bythe pre-registration roller pair 8 is long, the shearing force can bereleased to the loop space 19 before the sheet is buckled, so thatcreasing of the sheet can be prevented. Therefore, the presentembodiment is capable of corresponding to various media, such aslong-size sheet, small-size sheet, thin paper, thick paper and coatedpaper.

Modified Example

In the above-described embodiment, the guide surface 18 a of theconveyance guide 18 and the inclined surface 19 a of the bulged portion18 c are connected in a curved manner at the upstream end 18 b, butinstead of the conveyance guide 18, conveyance guides 128 and 138 may beformed as described below.

That is, as illustrated in FIG. 8A, the conveyance guide 128 composes acontact portion 128 b between the guide surface 18 a and the bulgedportion 18 c by a curved line of radius J, so that the guide surface 18a and the inclined surface 19 a are connected smoothly. Further, asillustrated in FIG. 8B, the conveyance guide 138 includes a convexportion 138 e that protrudes inward of the conveyance path CP and aconcave portion 138 d that protrudes outward of the conveyance path CP,and the inclined surface 19 a is formed continuously from the convexportion 138 e.

Even if conveyance guides 128 and 138 are formed in this manner, theintersection 32 is positioned within the loop space 19 and theintersection 33 is positioned outside the conveyance path CP. Thus, evenif shearing force acting on the sheet is increased, the shearing forcecan be released to the loop originating from the intersection 32 andbuckling and creasing of the sheet can be prevented.

According to the present embodiment, the pre-registration roller pair 8and the registration roller pair 15 are both composed of a pair ofrollers, but the present embodiment is not restricted thereto, and oneor the other of the rollers may be formed of a rotary member such as abelt. Further, the conveyance guides 18, 128 and 138 may each becomposed of a guide member divided into multiple parts.

All embodiments described above have been illustrated based on theelectro-photographic printer 201, but the present invention is notrestricted thereto. For example, the present invention can be applied toan inkjet image forming apparatus in which inks are ejected from nozzlesto form images on sheets.

Other Embodiments

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2017-166166, filed Aug. 30, 2017, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A sheet conveyance apparatus comprising: a firstrotary member pair comprising a first nip portion and configured toconvey a sheet; a second rotary member pair arranged downstream of thefirst rotary member pair in a sheet conveyance direction, comprising asecond nip portion and configured to convey the sheet being conveyed bythe first rotary member pair; and a conveyance guide configured to guidethe sheet having passed through the first nip portion to the second nipportion, and forming a curved conveyance path, the conveyance guidecomprising a first conveyance guide portion and a second conveyanceguide portion arranged downstream of the first conveyance guide portionin the sheet conveyance direction, the conveyance path including a loopspace in which a sheet abutted against the second nip portion is capableof forming a loop, wherein in the conveyance path, the first rotarymember pair and the second rotary member pair are arranged adjacently,the second nip portion is positioned upward than the first nip portionin a vertical direction and positioned at a position different from thefirst nip portion in a horizontal direction, the conveyance guide isarranged on an outer side of the curved conveyance path between thefirst nip portion and the second nip portion, the sheet conveyed by thefirst rotary member pair is abutted against the second nip portion so asto correct skewing of the sheet, if a common tangent of the first rotarymember pair at the first nip portion is referred to as a first straightline, a common tangent of the second rotary member pair at the secondnip portion is referred to as a second straight line, and a straightline that passes an intersection of the second straight line and theconveyance guide and has an inclination corresponding to the secondconveyance guide portion is referred to as a third straight line, thesecond straight line and the third straight line intersect each other,the loop space is arranged upstream of the intersection of the secondstraight line and the conveyance guide in the sheet conveyancedirection, a first intersection at which the first straight line and thethird straight line intersect is positioned within the loop space, and asecond intersection at which the first straight line and the secondstraight line intersect is positioned outside the conveyance path. 2.The sheet conveyance apparatus according to claim 1, wherein a minorangle formed by the first straight line and the third straight line isgreater than a minor angle formed by the second straight line and thethird straight line.
 3. The sheet conveyance apparatus according toclaim 1, wherein the first conveyance guide portion is bulged toward adirection away from the first straight line so as to form the loopspace, and the first straight line intersects with the second conveyanceguide portion.
 4. The sheet conveyance apparatus according to claim 3,wherein the second conveyance guide portion is configured to contact thesheet and receive load from the sheet in a state where the sheet isabutted against the second nip portion and sagged by its own weight. 5.The sheet conveyance apparatus according to claim 3, wherein a leadingedge of the sheet conveyed by the first rotary member pair is guided bythe first conveyance guide portion to the second conveyance guideportion, and then, guided by the second conveyance guide portion to thesecond nip portion.
 6. The sheet conveyance apparatus according to claim3, wherein the first conveyance guide portion is formed in a curvedshape and formed continuously with an upstream end, in the sheetconveyance direction, of the second conveyance guide portion, and thefirst intersection is arranged upstream than the upstream end of thesecond conveyance guide portion in the sheet conveyance direction. 7.The sheet conveyance apparatus according to claim 6, wherein the secondconveyance guide portion is a flat surface.
 8. The sheet conveyanceapparatus according to claim 7, wherein the second conveyance guideportion extends not to parallel with the first and second straightlines, and the first and second nips are positioned on one side withrespect to the third straight line.
 9. The sheet conveyance apparatusaccording to claim 1, wherein the loop space is greater than aconveyance space formed between the conveyance guide and the secondrotary member pair.
 10. An image forming apparatus comprising: the sheetconveyance apparatus according to claim 1; and an image forming unitconfigured to form an image on a sheet conveyed by the sheet conveyanceapparatus.